Shaped articles made of polycarbonate have already been known for some time. Polycarbonate sheets are known from EP A 0 110 221, for example, and are prepared for a large number of applications. The sheets are produced e.g. by extruding compositions that contain polycarbonate. Coextrusion with other compositions that contain polycarbonate and, in addition, a relatively high proportion of UV absorbers may optionally take place. However, polycarbonate has the disadvantage that it is not itself inherently UV-stable. The sensitivity curve of bisphenol A based polycarbonate exhibits the highest sensitivity between 320 nm and 330 nm. Below 300 nm, hardly any solar radiation reaches the earth and above 350 nm, the sensitivity of polycarbonate is so low that yellowing no longer takes place to any significant extent.
To protect polycarbonate from the harmful effect of UV rays in the atmosphere, UV stabilizers are generally employed, which absorb the UV radiation and convert it into harmless thermal energy.
It is advantageous for lasting protection if the harmful UV radiation is effectively filtered out before it reaches the polycarbonate surface, which is possible by using UV protective layers, e.g. coextruded layers containing UV absorbers, films containing UV absorbers or paints containing UV absorbers, on polycarbonate.
Another very important property is the protection of polycarbonate products, particularly polycarbonate sheets, from UV light in exterior applications. For this purpose, a polycarbonate outer layer in thicknesses of 10 to 200 μm, preferably 20 to 100 μm, particularly preferably 20 to 60 μm, containing relatively high concentrations of UV absorbers, generally of between 0.5 and 15 wt. % UV absorbers, is applied on to polycarbonate sheets (solid, corrugated and multi-wall sheets) in a coextrusion process.
EP A 0 320 632 describes coextruded sheets comprising compositions containing polycarbonate, which contain a UV absorber and may contain a lubricant. It is disadvantageous that, with a prolonged period of extrusion, the surface of the sheets is disadvantageously affected by evaporations from the melt of the composition, particularly in the case of coextrusion.
A recurring problem in the extrusion of these sheets is the settling out of volatile components from the composition on the calibrating unit (in the case of multi-wall sheets) or on the rollers (in the case of solid sheets), which may lead to surface defects on the sheets. Volatile components are e.g. UV absorbers, mold release agents and other low-molecular-weight components of the composition. The increased evaporation of the UV absorber from the melt of the coextrusion layer leads to the formation of a deposit on the calibrating unit or the rollers, and ultimately to the formation of defects on the surface of the sheets (e.g. white patches, waviness etc.). In addition, polycarbonate abrasion on the calibrating unit leads to powdery deposits on the polycarbonate sheets.
The conventional UV absorbers used are preferably selected from the group consisting of (bis[2-hydroxy-5-tert.-octyl-3-(benzotriazol-2-yl)phenyl]methane), 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-(hexyl)oxy phenol and 2-cyano-3,3-diphenylpropenoic acid 2,2-bis[(2-cyano-1-oxo-3,3-diphenyl-2-propenyl)oxy]-methyl-1,3-propanediyl ester.
The present invention is based on the object of, on the one hand, improving the coextrusion production process for the multi-layer products described in such a way that the cleaning intervals for the calibrating plates (multi-wall sheet extrusion) and rollers (solid sheet extrusion) are as great as possible; and on the other hand, improving the weathering resistance of the multi-layer products produced.
An improvement in weathering resistance is shown e.g. by a smaller increase in the yellowness index YI after artificial ageing.